Growers must know herbicides: Resistance now a concern in peanuts

Glyphosate and ALS herbicide resistance continues to be a major concern for Georgia growers, making it more important now than ever that they know the mode of action of the products they use, says Eric Protsko, University of Georgia Extension weed scientist.

“If you farm in Georgia, you know we're dealing with glyphosate and ALS resistance. When we talk about resistance, you need to know how your herbicides kill weeds — that will help you to manage your resistance problems,” said Protsko, speaking at the recent Georgia Peanut Farm Show in Albany.

Several counties in Georgia already have been or are being tested for pigweed resistance to glyphosate, he says. “It started in 2005 in three counties in middle Georgia. Over the years, we have found it more and more. Glyphostate resistance is real, it is a concern, and we have to do a better job of handling our glyphosate applications. Somewhere down the line, it'll be hard for us to get that product to work on some of these weeds that are resistant,” says Protsko.

This doesn't mean, he adds, that if a county has reported resistance, every field in that county has glyphosate-resistant pigweed.

Chances are good

“But if you've been using glyphosate in multiple crops and in multiple applications every year, that's when you're mostly likely to have it,” he says.

The other resistance concern is with the ALS herbicides, notes Protsko. “When I say ALS herbicides in peanuts, I'm talking about Cadre, Pursuit, Strongarm and Classic. They all have the same mode of action, and we do have resistance to those as well. If you're a cotton grower, and you're using Staple and Envoke, those are also ALS herbicides. And if you're a corn grower, Accent is an ALS herbicide.

“It's not only what you do with peanuts, but it's what you do in your other crops that affects how much resistance develops. As we go into the future, it's important for you to know your modes of action,” he says.

There are a lot of scientific names describing how herbicides kill plants, but it's not necessary for growers to know those, says Protsko. The updated version of the University of Georgia Pest Control Handbook will list “MOA” with a number next to it, he says. If the MOA numbers on two products match, then they have the same mode of action.

Turning to new products for peanut weed control, Protsko says there aren't many developments at the current time. “Late last year, we did get a new label for Aim in peanuts. This probably won't be used on a wide scale, but it'll help us in certain situations. The label is for using it as a harvest aide just before digging.

“I see this as useful in those occasional situations where you may have a bad morningglory problem. You know it can be a nightmare if you have ever tried to dig peanuts with morningglories. Aim will not remove the morningglories, but it will make them brittle, and it'll be easier to get that digger through the field so you don't have to get off every two minutes. It also might give you some activity on pigweed and tropical spiderwort,” he says.

Major concern

Palmer amaranth pigweed also is a major concern for Georgia peanut producers, says Protsko. A prolific seed producer, it can produce up to one million seeds under the right conditions.

“If you see one in the field, but you ignore it, a female plant can easily produce 500,000 seed. With such a buildup of seed, you can be back to ground zero very quickly. It also grows very rapidly, from 2 to 3 inches per day. If you go for a week without implementing a control strategy, that control strategy probably won't do you any good.”

Palmer amaranth is a very competitive plant, and only a few of the weeds can cause a lot of yield loss, says Protsko.

“The only way to control Palmer amaranth is to use a residual herbicide,” he says. “For peanuts, we have Valor, Dual Magnum, Outlook and the generic products. Strongarm will work on pigweed as well, but if they're ALS resistant, Strongarm won't work.”

Growers need to be more timely with their postemergence applications, says Protsko. “We can't allow those weeds to get 5 to 20 inches tall because we won't be able to control them effectively. If you're a cotton grower, you're being forced down a path where you'll have to use certain herbicides to manage pigweed. In cotton, you'll probably use Reflex in a lot of cases, and you'll use Valor in peanuts. Reflex and Valor are different herbicides, but they have the same modes of action. If we start over-using one of those two products, it's not inconceivable that we'll develop a resistance problem with them, and then we'll be in trouble because we'll lose some good tools.”

In comparing the residual products for controlling Palmer amaranth, Protsko says there isn't much difference in control for the first 20 to 30 days. “It's later when we see differences. In growers' fields with glyphosate-resistant pigweed, Valor still looked pretty good after 38 days. Dual Magnum gave good control for 30 days, but after that, it began to run out.”

If a grower is using a residual herbicide, there's not a guarantee it'll work unless it's activated by either rain or irrigation, he says. “If you have irrigation, you'll likely have more success. Dryland growers will have to rely on rainfall. We can incorporate products like Dual Magnum, Outlook and the generics, but we do not want to incorporate Valor mechanically.”

This past year was the first year that Provost fungicide was introduced into the market, says Protsko, and it is used in much the same manner as Folicur in a four-block program. “It's a good fungicide, but we had problems last year when we mixed it with something else — typically 2, 4-DB or 2, 4-D and some kind of fertilizer or surfactant. We were seeing leafspot and leaf shed, and a lot of this occurred when there were multiple tank-mixes in that situation Provost by itself didn't really cause a lot of problems. As a result of the experiences this past year, there will probably be some specific recommendations this year on what you can't mix with Provost.”

Tank-mix options

Many growers want to know, he says, what they can tank-mix with Provost. “It's a bad thing to mix three things in a tank, because we just don't know what'll happen. There are so many combinations out there, we could never test them under all environments to guarantee that you'll have crop safety, especially when it's 100 degrees F. and 99-percent humidity.”

Growers still have questions, says Protsko, about the relationship between Classic herbicide and tomato spotted wilt virus. Ten to 20 percent of Georgia peanuts are still treated with Classic for controlling Florida beggarweed, he says.

“There was some thought you might see an increase in tomato spotted wilt virus when we used Classic, so we did trials to see how much it affected the virus and how much it influenced yields. Over the past seven years, we've looked at six peanut varieties in 15 locations. One variety didn't respond any differently than another from the use of Classic. On the average, tomato spotted wilt increased by about 8 percent when we sprayed Classic. But we've never proven that Classic is causing yield loss. So if you think you have a bad enough problem with Florida beggarweed, spray Classic and don't worry about losing yield to tomato spotted wilt virus.”